1. Field of the Invention
This invention relates to batch-type apparatus for heating a workpiece by vapor phase or condensation heating.
2. Description of the Related Art
Condensation heat transfer is known to be extremely effective for heating a variety of articles for purposes of curing, soldering, brazing and the like. Typical condensation heating apparatus includes a chamber containing a heat transfer liquid and a heater near the bottom of the chamber to boil the liquid and establish a saturated vapor zone directly above the liquid. As a workpiece cooler than the boiling liquid enters the vapor zone, the vapor surrounds and condenses on external surfaces of the workpiece. During this phase change, the latent heat stored in the vapor is transferred to the workpiece, providing thermal energy in order to carry out the curing, soldering, brazing or other desired operation. Additional technical information relating to condensation heat transfer may be obtained from 3M Technical Bulletins No. 98-0211-2168-0.
Condensation heating apparatus for batch-type operations often has a chamber with an open top for receiving the article or workpiece to be processed. Conventional apparatus includes cooling coils extending horizontally around the perimeter of the chamber at a location directly above the desired upper boundary of the vapor zone and below the chamber top in order to establish and maintain a layer of cool air. In practice, most of the vapor remains in the vapor zone at the lower part of the chamber below the layer of cool air and does not rise and escape through the open top, since the heat transfer liquid that is normally selected yields a vapor substantially denser than air.
If, for some reason, the vapor zone is disturbed, a portion of the vapor moving past the horizontal plane of the perimeter cooling coils tends to condense on the sidewalls of the chamber and then descend toward the liquid at the bottom of the chamber. In addition, the cool air layer tends to reduce the amount of vapor that might otherwise escape, although such phenomena are most efficient in areas adjacent the sidewalls near the cooling coils. However, it is important to reduce the quantity of vapors escaping from the chamber as much as practicable in order reduce the cost of replenishing the heat transfer liquid.
A number of factors can affect usage rates of the liquid, including the power setting of the heater, the velocity of room air currents in the vicinity of the open-topped chamber, the configuration of the workpiece to be heated, and the quantity of condensation clinging to the workpiece as the latter is taken out of the vapor zone. Some factors, however, cannot readily be changed. For example, the chamber of the batch condensation heating apparatus is normally fixed in size and may be much larger than a size that would adequately heat smaller workpieces or only a few workpieces, resulting in inefficiency due to the possibility of escape of vapors in a middle portion of the chamber away from the cooling coils.
Recently, there has been increased interest in utilizing condensation heating apparatus for curing of polysiloxane coatings on plastic eyeglass lenses such as those made of polycarbonate materials. In the past, retail optical stores have not typically offered "one hour" service for prescription eyewear that includes a thermally cured abrasion resistant coating because such coatings are typically cured in an air oven for a period of 2 to 4 hours. Vapor condensation heating apparatus is useful for such retailers because the curing time of polysiloxane coatings on lenses may be significantly reduced to a time on the order of about four to forty minutes.